Can't select some vertices
It looks like you accidentally hid those faces/vertices.. If you open that model again, go into 'Edit Mode' then press 'ALT+H' you'll see that those vertices/faces were, maybe by mistake, hidden. 'H' hides anything that you have selected. and 'ALT+H' unhides them. If you selected those faces, verts or edges and accidentally pressed 'H' then you probably hid them so they'd remain hidden until you press 'ALT+H' to unhide them.